Powder injection system for detonation-operated projection gun

ABSTRACT

The powder injection system consists of a dosage chamber directly fed by a conventional powder feeder and communicated with barrel of the detonation gun through a direct conduit. In this way, the pressure wave advancing through barrel enters the communication conduit and upon reaching dosage chamber undergoes a sudden expansion, interrupting powder feeding from the continuous feeder and causing the full fluidization of powder contained in dosage chamber. The fluidized powder is suctioned to barrel where it remains until the pressure wave generated in a new detonation cycle carries it away, depositing it on the surface of the part to be coated.

TECHNICAL FIELD

This invention relates to the field of thermal spray technologies forapplying coatings, and in particular to detonation thermal spray.

BACKGROUND OF THE INVENTION

At this time, detonation spray technology is mainly used to applycoatings to workpieces exposed to severe wear, heat or corrosion and isfundamentally based on using the kinetic energy produced in thedetonation of a combustible mixture of gases to deposit powdered coatingmaterials on workpieces.

Coating materials typically used in detonation processes include powderforms of metals, metal-ceramics and ceramics and are applied to improveresistance to wear, erosion, corrosion, as thermal insulators and aselectrical insulators or conductors.

Spraying by detonation is performed by spray guns which basicallyconsist of a tubular detonation chamber, with one end closed and anotheropen, to the latter being attached a tubular barrel. A combustion gasmixture is injected into the detonation chamber and ignition of the gasmixture is achieved with a spark plug, causing a detonation andconsequently a shock or pressure wave which travels at supersonic speedsinside the chamber and then inside the barrel until it leaves throughthe open end of the barrel.

The coating material powder is generally injected into the barrel infront of the propagating shock wave front and is then carried out of theopen end of the barrel and deposited onto a substrate or workpieceplaced in front of the barrel. The impact of the coating powder onto thesubstrate produces a high-density coating with good adhesivecharacteristics.

This process is repeated cyclically until the part is adequatelycovered.

Powder feeders commercially available supply a continuous feeding whichmakes them adequate for high-velocity or plasma spray technologies.Since detonation is a discontinuous process, however, it requiresdiscontinuous powder feeding.

Feeders used in detonation devices provide discontinuous feeding byusing devices which control the amount of powder supplied to thedetonation barrel in each explosion. These devices, however, aredesigned specifically for each type of gun, that is, they cannot beinterchanged for use with other guns or in other machines which requirefeeding powder.

With respect to the powder measuring system employed. they can beclassified in two categories:

a) Mechanical: These devices use moving mechanisms (valves, spindles,gears, etc.) to introduce constant quantities of powder in eachdetonation cycle. Devices of this type are described for example in U.S.Pat. NO. 3,109,680, and in European Patent 0 484 533.

These devices have the main advantage of providing precise measurementsbut are however of great complexity they have many components. Theirreliability is low since they require periodic maintenance to maintainthe precision of the measurement. Finally their productivity is lowsince they is are limited to low operation frequencies.

b) Pneumnatic: These devices use gas pulses synchronised with thedetonation pulses to introduce the powder cyclically in the detonationbarrel, with these pulses sometimes being obtained from the detonationprocess itself. The elegance and mechanical simplicity of these deviceshas contributed to their wide use despite their precision beingquestioned. There are also numerous Patent documents such asPCT/US96/20129 by the same authors.

These devices share the characteristic of incorporating a volume ordeposit in which a limited amount of powder is stored, which, bygravity, feeds another volume or dosage chamber which feeds thedetonation barrel by a gas impulse. The disadvantage of these systems istheir lack of precision in the amount of powder dosed, mainly due totheir difficulty, over long spray periods, of keeping stable the volumeand/or pressure of the feeding deposit. This is due to the fact thatpart of the detonation wave enters the powder feeding deposit,pressurizing it so that the powder falls under gravity and due to thepressure existing in the deposit at each time.

In addition, since the amount of powder entering the dosage chambercannot be perfectly controlled, the degree of fluidization produced bythe impulse gas cannot be controlled either, and thus it is difficult toknow precisely the amount of powder injected into the barrel.

Furthermore, since in these devices feeding from the deposit to thedosing chamber is by gravity, when the detonation gun generally handledby an industrial robot) assumes positions in which the powder deposit isnot vertical, the powder will not fall into the dosage chambercontinuously. Thus, it is difficult to ensure a constant feeding.

Document GB-A-2 192 815 is known in prior art, which describes adetonation coating device comprising a barrel open at one end, a gasfeeding system, a blast initiating assembly and a powder bath meteringunit consisting of a vertically oriented bunker changing at its lowerpart into a vertical tube under which, inside the barrel, a horizontalrack is located. The barrel is oriented vertically with its axisparallel to the axis of the bunker, whereas the tube is connected to thebarrel through the closed butt-end of the latter.

In British Patent GB-2192815, the deposit containing the powder to bedischarged is placed vertical, with the powder falling on thedistribution tray under the action of gravity, which moans that the guncan only operate in positions where the deposit, the distribution ductand the tray are arranged vertically, as otherwise the powder would notbe supplied. Thus, this gun cannot be used mounted on a robot arm as thelatter's motion would be limited by the position of the powder deposit.

The powder is fed from a closed deposit, so that as the deposit isemptied, conditions inside it change, particularly the temperature andpressure. Thus it is not possible to ensure a control of the amount ofpowder introduced.

The dosing of the powder to be used in each blast cycle is determined bysize and arrangement of he distribution tray, and is interrupted whenthe powder reaches a height in the tray which obstructs the outlet ofthe distribution duct, so that the gas carries the amount of powderpresent in the tray. Thus, there is not an exact control of the dosedamount as the tray may be more or less filled depending on the chamberconditions and on the powder.

The powder feeder is on a fixed position on the rear wall of thecombustion chamber, so that it is only suitable for performing certaintypes of coatings. This is so because, depending on the type of coatingdust employed, a specific barrel length is required, and as the dustfeeder is on the rear wall the length of the gun will always be thesame. Thus, for coatings which require different barrel lengths we wouldneed a different gun, suitable for this coating. The gun of GB-2192815is therefore quite inflexible as regards the coatings which may beobtained.

This detonation coating device is not suitable for providing goodcoatings with any kind of materials, but it is only appropriate forparticular coatings.

SUMMARY OF THE INVENTION

The present invention fully solves the above disadvantages by using aninjection system which allows employing a conventional type continuouspowder feeder for feeding a detonation spray system, the powderinjection being performed cyclically, in synchronization with the gunspray frequency and with great precision in the powder dosage.

The system proposed allows directly connecting the gun and thecontinuous powder feeder and consists of a dosage chamber which receivesthe continuous powder feeding and a conduit which directly communicatesthe chamber with the gun barrel. Consequently, in each detonation cycle,the detonation pressure wave reaches the dosage chamber, momentarilyinterrupting the feeding so that the ensuing suction of the detonationwave carries the powder contained in the dosage chamber,therebyinjecting it into the gun barrel.

With this object the dosage chamber communicates with the gun barrel bya direct tubular conduit of small diameter, so that the pressure wavethat advances through the barrel passes to the communication conduit andon reaching the dosage chamber undergoes a sudden expansion which fillsthe chamber with pressurized gas, blocking the entry of the powderfeeding conduit. In this way, the feeding of powder from the continuousfeeder is cyclically interrupted, and it is therefore possible todetermine the exact amount of powder present in the dosage chamber atthe time of detonation.

The sudden expansion of the gas in the dosage chamber creates aturbulence which produces the fluidization of all the powder containedin the dosage chamber so that the suction process, which follows thedetonation, carries all the powder contained in the chamber, so that itis possible to control the exact amount of powder injected into thebarrel. In addition, as the pressure wave is composed of hot gasesproduced in the combustion process the interaction of these gases withthe powder contained in the dosage chamber produces a preheating of thepowder which favors its fluidization.

In this way, when the pressure wave generated in the detonation passesthe communication conduit of the dosage chamber, the low pressuregenerated after the detonation wave creates a suction which carries thegas contained in the dosage chamber and the fluidized powder. The powdercarried reaches the barrel, where it remains until the pressure wavegenerated in the following detonation cycle carries it, depositing it onthe surface of the part to be coated.

With this injection system the pressure wave from the detonation is madeto perform the injection of powder into the barrel cyclically andsynchronized with the gun firing frequency, thus transforming acontinuous powder feeding into a pulsed injection to the gun barrelwithout using complex mechanical devices.

In addition, the expansion created by the dosage chamber reduces thevelocity of the pressure wave preventing it from eroding the dosagechamber and advancing into the powder feeder, eliminating the risk of itproducing irreparable damages to the feeding system.

The dosage chamber presents an elongation or auxiliary chamber oppositethe communication conduit to the detonation barrel which is meant toincrease the length of the dosage chamber to reduce the force of theimpact and therefore the effects of the erosion produced by theencounter of the gases and the powder in this area of the dosagechamber.

The device of the invention presents the following advantages:

It favors a cyclical interruption of the feeding by the detonationpressure wave.

It favors a preheating and fluidization of the powder by its interactionwith the hot gases of the combustion.

It allows feeding a precise amount of powder in each explosion by thesuction effect which follows the pressure wave in each detonation.

BRIEF OF THE DRAWINGS

To complement the description being made and in order to aid a betterunderstanding of the characteristics of the invention, attached to thepresent descriptive memory and as an integral part of the same is a setof drawings where, with an illustrative and non-limiting nature, thefollowing has been shown:

FIG. 1 shows a sketch of the powder injection device of the invention.

FIG. 2 shows an operation sequence of the powder injection device of theinvention.

FIG. 3 shows a graph showing the evolution of pressure at the powderinjection point along two firing cycles of the detonation gun.

FIG. 4 shows a sketch of the embodiment with a double powder injectiondevice.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As shown in FIG. 1 the system of the invention is a connection devicebetween a continuous feeding system and a detonation gun and basicallyconsists of an expansion and dosage chamber (2) which 10 is reached by adirect conduct (5) by the powder supplied by a continuous feeding system(7), not shown, the dosage chamber (2) being connected to the barrel (1)by a direct conduit (4).

The dosage chamber (2) is basically an expansion chamber whichcommunicates with the barrel (1) of the gun through a direct tubularconduit (4) of reduced diameter, so that the pressure wave advancingthrough the barrel (1) passes to the communication conduit (4) andreaches the dosage chamber (2). The detonation gases which reach thedosage chamber (2) undergo a sudden expansion which fills the chamberwith gas, blocking the entry of the powder feeding conduct (5). In thisway it is possible to cyclically interrupt the feeding of powder fromthe continuous feeder (7) and thus it is possible to control the amountof powder dosed in the chamber and consequently the amount of powderinjected to the barrel in each detonation cycle.

The sudden expansion of the gas in the dosage chamber (2) creates aturbulence which produces the fluidization of all the powder containedin the dosage chamber (2), so that the auction process which follows thedetonation carries all the powder contained in the chamber injecting itinto the barrel (1). The fluidization of the powder contained in thedosage chamber (2) is favored by the fact that the gases of thedetonation wave are at a high temperature.

In this way, when the pressure wave, generated by the detonation, passesthe communication conduct (4), the low pressure generated after thedetonation wave produces a suction which carries the gas contained inchamber (2) and the powder included in it (which is totally fluidized).The powder is carried to the barrel (1) where it remains until thepressure wave produced in a new detonation cycle carries it, depositingit on the substrate (3) or part to be covered. (See FIG. 2)

In addition, the expansion of gases of the detonation wave insidechamber (2) produces a reduction in their velocity, minimizing theerosion effect on the chamber (2) walls and preventing the pressure wavefrom advancing through conduit (5) to the powder feeding system (7).

Although expansion chamber (2) reduces the speed of the pressure wave,unavoidably there is interaction between the gases and the inner wallsof the chamber in the area opposite the communication conduit (4) sothat the impact of the pressurized gas and the fluidized powder againstthis area would inevitably result in severe erosion. For this reason,the dosage chamber is provided with an extension or auxiliary chamber(6) with an inlet point opposite communication conduit (4) so that thepressure shock wave expands inside the dosage chamber (2) and inside theextension (6) avoiding a violent collision of the shock wave with thewalls of chamber (2).

The expansion chamber (2) can have any shape or size as long as thegases which enter it through conduit (4) undergo a sudden expansion asthey enter the chamber. Communication conduit (4) can also have anylength or diameter as long as it is great enough so that the powder doesnot adhere to the conduit walls, blocking it, and so that the pressureof the detonation wave which travels through the conduit (4). is not toolarge, that is, as long as the pressure allows fluidization of thepowder contained in the chamber but does not endanger the continuouspowder feeding system nor exhaust the energy available for detonation.

FIG. 3 shows a graph with the pressure variations with time at thepowder injection point. A peak or sudden pressure increase (D) can beclearly seen, corresponding to the detonation, followed by a pressuredrop (S) corresponding to the suction following the detonation. Thepressure then remains more or less constant until the following cyclewhen a new pressure peak (D) occurs, followed by the ensuing suction(S).

With this configuration, as seen in FIGS. 2 and 3, the operationsequence corresponding to a gun operation cycle with the injector of theinvention will be the following:

A conventional continuous powder feeding system (7) supplies powder tothe dosage chamber (2) via a conduit (5). This feeding occurscontinuously and directly, without any valves or closing mechanismsbetween the powder feeding system (7) and the dosage chamber (2).

When the pressure wave (D) front reaches the communication openingbetween conduit (4) and barrel (1) part of the detonation gases enterthrough conduit (4) until they reach the dosage chamber (2). On reachingthe dosage chamber (2), these gases undergo a sudden expansion whichfills the dosage chamber (2) with pressurized gas, blocking entry ofpowder from conduit (5), and thereby converting the continuous powderfeeding into a discontinuous filling of the dosage chamber.

In addition, the sudden expansion of gases generates a turbulence whichcauses the fluidization of all powder contained in the dosage chamber(2), the fluidization being favored by the high temperature of thedetonation gases.

Once front (D) of the detonation wave has fully passed the communicationorifice to the conduit (4), low pressure (S) causes a suction whichcarries the gases contained both in the dosage chamber (2) and inconduit (4), and therefore, the powder contained in the dosage chamber(2). In this way, the powder reaches the barrel, to await the followingpressure front (D) that corresponds to the following detonation, whichwill carry the powder away with it. As all the powder contained in thedosage chamber (2) is fluidized, the suction generated by the pressurewave carries all the powder in the dosage chamber (2), thus obtaining aperiodic and controlled injection of powder into the barrel.

Finally, FIG. 4 shows a double device consisting of two injectionsystems in order to allow feeding of different types of powders atpoints axially separated from the barrel to obtain multiple-layercoatings or even coatings of gradient composition.

The powder injection apparatus of the present invention, whenincorporated to a detonation system, increases its precision,reliability, versatility, and productivity as compared to conventionalsystems.

What is claimed is:
 1. A powder injection system for a detonation spraygun of the type comprising a gas supply, an ignition source, and abarrel, the system comprising: an expansion and dosage chamber directlyfed by a continuous powder feeding device; and means for communicatingwith the dosage chamber and the barrel, the means being disposed so thata first pressure associated with gases traveling down the barrel, whenreaching the dosage chamber, temporarily interrupts powder feeding untila subsequent lower second pressure sucks powder contained in the chamberto the barrel.
 2. A powder injection system for a detonation spray gunas claimed in claim 1, wherein the communication conduit has a reduceddiameter, so that the gases which advance through the communicationconduit undergo expansion on reaching the dosage chamber.
 3. A powderinjection system for a detonation spray gun as claimed in claim 1,wherein the carrier gas supply is a continuous supply.
 4. A powderinjection system for a detonation spray gun as claimed in claim 1,wherein the communication conduit has a sufficiently large diameter toprevent powder from adhering to inner walls of the communicationconduit.
 5. A powder injection system for a detonation spray gun asclaimed in claim 1, wherein the barrel has an axis and the conduit isdisposed radially about the axis.
 6. A powder injection system for adetonation spray gun as claimed in claim 1, wherein the powder feedingdevice further comprises a carrier gas supply.
 7. A powder injectionsystem for a detonation spray gun of the type comprising a gas supply,an ignition source, and a barrel, the system comprising: an expansionand dosage chamber directly fed by a continuous powder feeding device,wherein the expansion and dosage chamber incorporates an extension orauxiliary chamber which increases the length of the dosage chamber; acommunication conduit in communication with the dosage chamber and thebarrel, wherein the conduit is disposed so that a first pressureassociated with gases traveling down the barrel, when reaching thedosage chamber temporarily interrupts powder feeding until a subsequentlower second pressure sucks powder contained in the chamber to thebarrel.
 8. A powder injection system for a detonation spray gun asclaimed in claim 3, wherein the auxiliary chamber is in fluidcommunication with the dosage chamber at a point opposite thecommunication conduit.
 9. A method for introducing powder to adetonation spray gun of the type comprising detonation gases supply, anignition source, and a barrel having an open end, the method comprising:feeding powder into an expansion and dosage chamber; feeding detonationgases to a barrel; igniting the detonation gases to produce a detonationpulse; passing a portion of the detonation pulse through a communicationconduit to the expansion and dosage chamber; and interrupting thefeeding of powder into the chamber and drawing the powder from thechamber into the barrel with said portion of the detonation pulse.
 10. Amethod for introducing powder to a detonation spray gun as claimed inclaim 9, wherein feeding detonation gases further comprises feedingdetonation gases through a communication conduit in fluid communicationwith the barrel.
 11. A method for introducing powder to a detonationspray gun as claimed in claim 9, wherein feeding powder into the chamberfurther comprises using a powder feeding device to feed powder into thechamber.
 12. A method for introducing powder to a detonation spray gunas claimed in claim 9, further comprising fluidising the powder in thechamber.
 13. A method for introducing powder to a detonation spray gunas claimed in claim 9, wherein feeding powder into the chamber furthercomprises using a carrier gas.
 14. A method for introducing powder to adetonation spray gun of the type comprising detonation gases supply, anignition source, and a barrel having an open end, the method comprising:feeding powder into an expansion and dosage chamber; feeding detonationgases to a barrel; igniting the detonation gases to produce a detonationpulse; passing a portion of the detonation pulse through a communicationconduit to the expansion and dosage chamber, wherein the communicationconduit is in fluid communication with the expansion and dosage chamberand tho barrel, wherein the portion of the detonation pulse in thechamber interrupts the feeding of powder into the chamber, and drawingthe powder from the chamber into the barrel, wherein the feeding issolely controlled by tho detonation pulse.
 15. A method for introducingpowder to a detonation spray gun as claimed in claim 14, wherein theinterruption of feeding causes cyclical feeding of the powder to thechamber.
 16. A method for introducing powder to a detonation spray gunas claimed in claim 15, wherein the igniting the detonation gases has afrequency and wherein the cyclical feeding is synchronized with thefrequency.